Monolayers phospholipid, ITIES

A theoretical approach based on the electrical double layer correction has been proposed to explain the observed enhancement of the rate of ion transfer across zwitter-ionic phospholipid monolayers at ITIES [17]. If the orientation of the headgroups is such that the phosphonic group remains closer to the ITIES than the ammonium groups, the local concentration of cations is increased at the ITIES and hence the current observed due to cation transfer is larger than in the absence of phospholipids at the interface. This enhancement is evaluated from the solution of the PB equation, and calculations have been carried out for the conditions of the experiments presented in the literature. The theoretical results turn out to be in good agreement with those experimental studies, thus showing the importance of the electrostatic correction on the rate of ion transfer across an ITIES with adsorbed phospholipids. 

Koryta et al. [48] first stressed the relevance of adsorbed phospholipid monolayers at the ITIES for clarification of biological membrane phenomena. Girault and Schiffrin [49] first attempted to characterize quantitatively the monolayers of phosphatidylcholine and phos-phatidylethanolamine at the ideally polarized water-1,2-dichloroethane interface with electrocapillary measurements. The results obtained indicate the importance of the surface pH in the ionization of the amino group of phosphatidylethanolamine. Kakiuchi et al. [50] used the video-image method to study the conditions for obtaining electrocapillary curves of the dilauroylphosphatidylcholine monolayer formed on the ideally polarized water-nitrobenzene interface. This phospholipid was found to lower markedly the surface tension by forming a stable monolayer when the interface was polarized so that the aqueous phase had a negative potential with respect to the nitrobenzene phase [50,51] (cf. Fig. 5). 

Capacitance measurements of phospholipid monolayers at the ITIES have been proposed as a suitable tool for studying the enzyme activity under the precise control of the electrical state of the monolayer [81]. Kinetics of hydrolysis of phosphatidylcholine... 

When a monolayer of phospholipids is adsorbed at the ITIES, there must be a modification of the electrical structure of the interface [60]. Since we aim at describing the effect of this monolayer on the rate of ion transfer in a simple way, we assume a sharp interface also in the presence of phospholipids. The hydrophobic tails are located in the organic phase (negative x region), and the hydrophilic headgroups are located in the aqueous phase (positive X region). 

To check the predictions of the Marcus theory of ET (6), one has to carry out measurements at higher overpotentials. This can be done by separating the electron donor and acceptor by a well-defined spacer (32), which decreases the ET rate. Molecular monolayers of long-chain saturated phospholipids (Fig. 8) have been used as a spacer in ITIES studies (26,27). 

FIG. 8 The ITIES modified with a monolayer of phospholipid. The insert shows the structure of synthetic saturated phosphatidyl choline lipid. (From Ref. 26.)... 

The kf versus [lipid] dependencies for Fe(CN)s are different. Although the ET rate decreases markedly with increasing concentration of lipid, it does not approach zero at higher concentrations, but reaches the limiting value at about 50 /xM. This saturation suggests the formation of a complete phospholipid monolayer at the ITIES. The formation of compact phospholipid monolayers at similar lipid concentrations was observed at water/nitro-benzene (40a) and water/dichloroethane (40b) interfaces. 

To analyze ion adsorption at the phospholipid monolayer at an ITIES, one needs to set up a thermodynamic model. In 2003, Samec et al. proposed a simple model comprising the adsorption of the zwitterionic form L , the formation of the cationic complex RL+ with an aqueous cation R+, and the desorption of the complex in the organic phase and its dissociation in the organic phase [329]. 

In summary, it is clear that ITIES provide a unique support to study phospholipid adsorption and the interaction of the phosphatidyl moiety with aqueous cation. It has been observed many times that a compact monolayer can hinder the transfer of some cations such TEA+, but does seem to be an effective barrier to the transfer of anions. Finally, charge-transfer studies combined to electrocapillary data have clearly shown that phosphatidylcholine acts as a strong ionophore for alkali metal cations and peptides. 

Similar to the earlier studies at metal electrodes, molecular monolayers of long-chain saturated phospholipids assembled at the ITIES have been used as spacers to check the predictions of Marcus theory (Figure 8.7) [28,29]. 

FIGURE 8.7 The ITIES modified with a monolayer of phospholipid. The insert shows the structure of synthetic saturated phosphatidylcholine lipid. (Reprinted with permission from Tsionsky, M., Bard, A. J and Mirkin, M.V., Long-range electron-transfer through a lipid monolayer at the liquid/liquid interface, J. Am. Chem. Soc., 119,10785-10792,1997. Copyright 1997 American Chemical Society.)... 

Amphiphilic lipid molecules dissolved in an organic solvent spontaneously form a monolayer film at the water-organic solvent interface. The orientation of the monolayer is such that the hydrophilic head group is immersed in water, while the hydrophobic tail remains in the organic phase. The first attempt to probe ET across a phospholipid monolayer adsorbed at the ITIES was reported by Cheng and Schiffrin, who found that a monolayer makes the ET rate immeasurably slow [53]. 

The blocking effect of the lipid film can be attributed to the decrease in either the ET or IT rate. Kakiuchi and coworkers [54] and Schiffrin and coworkers [55] extensively studied the effect of different lipid monolayers on transfer rates of various ions, including C104 . The largest decrease in IT rate caused by the presence of a compact phospholipid monolayer at the ITIES was by a factor of 5. The effects observed in Ref. [28] were orders of magnitude larger. This observation and also the characteristic shape of approach curves discussed in Section 8.2.1.3 allowed the authors to rule out the possibility of the slow IT effect. 

To prove the existence of separate domains in the mixed monolayer, the tip was scanned laterally above the interface. At a clear ITIES or the interface covered with a monolayer of a single phospholipid (either DPHC or C-16), the current fluctuations during the lateral scan did not exceed... 

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